1. Field of the Invention
The present invention relates to an optical disc device, and in particular, to the tracking control and the focus control of the optical disc device.
2. Description of the Related Art
Techniques as background arts of this technical field have been disclosed in JP-A-2002-312958, JP-A-2009-140568, JP-A-2001-357542 and JP-A-2009-170035, for example. These Patent Literatures have disclosed a disc having a plurality of data layers and a servo-dedicated layer used for precisely positioning a beam spot on each data layer, an objective lens control method for precisely condensing (focusing) laser spots on the servo layer and the data layer, correction of optical axis shifts of optical beams condensed on the servo layer and the data layer by use of error signals generated from returning light from the servo layer and the data layer, correction of optical axis tilts by use of error signals and shift error signals generated from returning light from the servo layer and the data layer, etc.
Meanwhile, JP-A-63-26814 disclosing a head positioning technique for a magnetic disc device includes a description “In cases where a medium defect exists in a prescribed area of the data surface when the head positioning is executed based on servo information recorded on the servo surface and the head positioning is executed again based on servo information recorded in a prescribed area of the data surface, . . . the medium defect is detected, the head positioning control based on the data-surface servo information is stopped, and the control is returned to the positioning operation based on the servo-surface servo information.” (see “Summary” in JP-A-63-26814).